that's depend on your system..
let's see..
a system equipped with
Processor: intel i7 3960X EE => Six core intel cpu with 130Watt TDP.
motherboard: Rampage IV Extreme
storage: Corsair SSD
graphic cards: 2x Sapphire Radeon R7 260X 2GB OC => note this one is OC factory edition..
[http://www.tweaktown.com/reviews/6617/sapphire-radeon-r7-260x-2gb-oc-in-crossfire-video-card-review/index3.html]
The system above only consume 275 watt when idle and 474 watt while stressing with 3d mark.
Even cx series is not best PSU, but you still have plenty headroom to run your crossfire setup in your computer with CX600M..